The present invention relates to a reactive or consumable anode made of an aluminum-based alloy for cathodic protection in seawater of iron, steels, and alloys susceptible to corrosion and embrittlement by hydrogen.
It is usually considered that corrosion is negligible with an iron concentration less than or equal to 10.sup.-6 mole/liter, i.e. an embrittlement potential less than or equal to -850 mV referred to the potential of a saturated calomel electrode (SCE). The potential range encountered for cathodic protection of ships in seawater and of offshore structures is between -850 mV and -1100 mV.
At these potentials, the use of known reactive protective anodes fully protects steel against generalized corrosion and corrosion by galvanic coupling which may occur in seawater. However, reduction of deaerated water causes hydrogen to be released at the surface of the steel thus protected, which may bring about additional corrosion. It is known that hydrogen embrittles certain steels with a high yield strength and their welds as well as titanium alloys, and that susceptibility to corrosion under stress due to hydrogen decreases sharply at potentials above -800 mV.
The performances and electrochemical characteristics of such an anode, which are principally the electrochemical potential at zero current, the current flowing per unit area at a given potential, the electrochemical efficiency, and the mass energy or quantity of current flowing per unit weight of dissolved anode in ampere-hours per kilogram, are determined by the alloy from which the reactive anode is made.
Aluminum and zinc alloys for making anodes protecting a metal structure in contact with an aggressive electrolytic medium are already known. Patent FR 2,377,455 describes compositions of alloys having aluminum or zinc percentages by weight of 8 to 40%, the remainder being zinc or aluminum. Aluminum-based alloys with added zinc have iron, silicon, and copper impurities and must have a purity of at least 99.80%. Stabilizers of the electrode potential such as mercury, indium, manganese, and titanium can be added.
Patent FR 2,449,730 discloses an aluminum-based protective alloy composition containing gallium in the proportions of 0.005 to 3.5 wt. % and magnesium in the proportions of 0.1 to 1 wt. %, and having known electrochemical properties.
An aluminum alloy for offshore protection containing 0.04 wt. % mercury and 2 to 4.5 wt. % zinc, with iron, silicon, and titanium impurities and a high mass energy of 2790 ampere-hours per kilogram is also known. This alloy is effective at a potential less than -1045 mV/SCE, with a current density of 1.5 mA/cm.sup.2.
An offshore protection aluminum alloy having 0.02 wt. % indium and 5 wt. % zinc, with less mass energy than the previous alloy, is also known.
Patent FR 2,616,806 describes an aluminum-based alloy composition containing indium weight percentages of 0.005 to 0.05, zinc of 0.05 to 8, gallium of 0.003 to 0.05, manganese of 0.01 to 0.3, iron of 0.03 to 0.3, and magnesium of 0.02 to 2 and silicon of 0.03 to 0.4.
These various known alloy compositions are effective against generalized corrosion and corrosion by galvanic coupling in the relatively low electronegative potential range of -1000 to -1100 mV but do not protect from corrosion under stress by hydrogen embrittlement, which is avoidable only at potentials greater than -800 mV. Steels are thus not fully protected from corrosion at electrode potentials less than -860 mV/SCE by reactive anodes having known aluminum-based alloy compositions.